Continuously tunable photonic crystal drop filter

Details Continuously tunable photonic crystal drop filter Continuously tunable photonic crystal drop filter

2002-02-27

2004-07-06

Stahl, Mike (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling function

C385S050000, C385S025000, C385S044000

Reexamination Certificate

active

06760514

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to the field of photonic crystals; and, more particularly, to a photonic crystal drop filter and to a method for tuning the transmission wavelengths of a photonic crystal drop filter.
2. Description of Related Art
Wave division multiplexing is a process that permits the transmission capacity of an optical communications system to be increased. In particular, in a wave division multiplexer (WDM) system, information is transmitted using a plurality of optical carrier signals, each carrier signal having a different optical wavelength. By modulating each carrier signal with a different one of a plurality of information signals, the plurality of information signals can be simultaneously transmitted through a single waveguiding device such as a single optical fiber.
For a WDM system to function properly, the system must have the capability of extracting a carrier signal at a certain wavelength from one waveguide and adding the signal at that wavelength to another waveguide so as to redirect the path through which the extracted carrier signal travels.
FIG. 1
is a block diagram that schematically illustrates components of a WDM communications system. The system is generally designated by reference number
10
, and includes a signal source
12
that transmits a plurality of carrier signals at different optical wavelengths through an optical fiber or other waveguiding device
14
. The optical fiber
14
is connected to an extraction device
16
that is capable of extracting one or more of the carrier signals carried by the optical fiber
14
and redirecting the extracted signal or signals to another optical fiber or waveguiding device
18
. The remaining carrier signals carried by the optical fiber
14
are transmitted through the extraction device
16
to an optical fiber
20
or the like. The carrier signals carried by optical fibers
18
and
20
are then further processed by processing structure not illustrated in FIG.
1
.
A practical WDM communications system must be capable of simultaneously transmitting a large number of carrier signals; and, therefore, must be able to carry a large number of light wavelengths. In the future, WDM systems will be required to carry even more carrier signals than today. The number of wavelengths that can be extracted by known extraction devices, however, is finite; and only a distinct set of wavelengths can be derived from any particular extraction device design. Furthermore, he wavelength separation of the carrier signals will be less in future WDM systems; and known extraction devices do not have the resolution that will be required to selectively extract the more closely spaced signals.
For example, drop filters are commonly used in optical communications circuits to extract light of a particular wavelength from one waveguide and direct the extracted light to another waveguide. In effect, a drop filter allows light of one wavelength to be dropped from one path in an optical communications circuit and added to another path in the circuit.
Known drop filters, however, can be designed to extract and redirect only a few distinct, well-separated wavelengths. Accordingly, known drop filters are not fully satisfactory for use as an extraction device in a WDM system that requires the capability of extracting carrier signals carried by light having a large number of different wavelengths.
Photonic crystals (PC) are periodic dielectric structures that can prohibit the propagation of light in certain frequency ranges. More particularly, photonic crystals are structures that have spatially periodic variations in refractive index; and with a sufficiently high refractive index contrast, photonic bandgaps can be opened in the structure's optical transmission characteristics. (The term “photonic bandgap” as used herein and as is commonly used in the art is a frequency range in which propagation of light through the photonic crystal is prevented. In addition, the term “light” as used herein is intended to include radiation throughout the electromagnetic spectrum, and is not limited to visible light.)
A photonic crystal that has spatial periodicity in three dimensions can prevent the propagation of light having a frequency within the crystal's bandgap in all directions; however, the fabrication of such a structure is often technically challenging. An alternative is to utilize a two-dimensional photonic crystal slab that has a two-dimensional periodic lattice incorporated within it. In a two-dimensional photonic crystal slab, light propagating in the slab is confined in the direction perpendicular to a major surface of the slab via total internal reflection, and light propagating in the slab in directions other than perpendicular to a major surface is controlled by the properties of the photonic crystal slab. A two-dimensional photonic crystal slab has the advantage that it is compatible with the planar technologies of standard semiconductor processing; and, in addition, the planar structure of the slab makes an optical signal in a waveguide created in the slab more easily accessible to interaction. As a result, a two-dimensional photonic crystal slab is susceptible to being used to create active devices.
FIG. 2
is a schematic, perspective view of a two-dimensional photonic crystal slab that is known in the prior art; and is provided to assist in explaining the present invention. The photonic crystal slab is generally designated by reference number
30
, and comprises a slab body
32
having a two-dimensional periodic lattice comprising an array of posts
34
therein. As shown in
FIG. 2
, the posts
34
are oriented parallel to one another and extend through the slab body from top face
36
to bottom face
38
thereof.
The two-dimensional photonic crystal slab
30
can take various forms. For example, the posts
34
can comprise rods formed of a first dielectric material, and the slab body
32
can comprise a body formed of a second dielectric material that differs in dielectric constant from that of the first dielectric material. Alternatively, the posts can comprise holes formed in a slab body of dielectric material; or the posts can comprise rods of dielectric material and the slab body can be air, or another gas, or a vacuum. In addition, the posts can be arranged to define a square array of posts; or they can be arranged in a different manner, such as in a rectangular array or a triangular array.
In a two-dimensional photonic crystal slab such as illustrated in
FIG. 2
, light propagating in the slab is confined in the direction perpendicular to the slab faces
36
and
38
via total internal reflection. Light propagating in the slab in directions other than perpendicular to the slab faces, however, is controlled by the spatially periodic structure of the slab. In particular, the spatially periodic structure causes a photonic bandgap to be opened in the transmission characteristics of the structure within which the propagation of light through the slab is prevented. Specifically, light propagating in the two-dimensional photonic crystal slab
30
of
FIG. 2
in directions other than perpendicular to a slab face and having a frequency within a bandgap of the slab will not propagate through the slab; while light having frequencies outside the bandgap is transmitted through the slab unhindered.
It is known that the introduction of defects in the periodic lattice of a photonic crystal allows the existence of localized electromagnetic states that are trapped at the defect site, and that have resonant frequencies within the bandgap of the surrounding photonic crystal material. By arranging these defects in an appropriate manner, a waveguide can be created in the photonic crystal through which light having frequencies within the bandgap of the photonic crystal (and that thus would normally be prevented from propagating through the photonic crystal) is transmitted through the photonic crystal.
FIG. 3
is a schematic, cross-sectional view that illustra

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